Vehicles operated with underinflated tires pose a significant safety problem. A major contributor to the high rate of tire failures due to underinflation and gradual pressure loss is that many people do not know whether or not their vehicle's tires are properly inflated. The U.S. Congress has mandated that automobile manufacturers implement a tire pressure monitoring system by 2007 for automobiles operating the in the U.S.
A typical tire pressure monitoring system includes a sensor/transmitter at each wheel to monitor pressure in the tires and a receiver inside the vehicle. Each sensor periodically transmits its unique ID and the pressure in the associated tire to the receiver. The received pressure value is compared to an acceptable pressure level and, if the measured pressure is outside acceptable parameters, a warning is issued to the driver.
Tire pressure monitoring systems can be classified as two types: wheel speed based, or indirect, systems and pressure sensor based, or direct, systems. Wheel speed based systems infer tire pressures using the vehicle's anti-lock braking system wheel speed sensors to measure tire-to-tire differences in rotational velocities that indicate that one tire is at a different pressure from the others. Pressure sensor based systems directly measure tire pressures with pressure sensors mounted either inside the tire or on the stem valve. For a variety of reasons, pressure sensor based systems are preferable.
However, currently available pressure sensor based systems do have some disadvantages. For example, pressure sensor based systems that are mounted inside the tire can suffer from signal degradation due to the interference caused by steel elements used in the tire structure.